In order to implement such a drilling process in this type of application, use is generally made of a machine tool comprising:
a casing;
a tool-holder spindle extending along an axis;
a mechanism for driving the tool-holder spindle, the mechanism comprising:                a first member for driving the spindle in rotation about its axis relative to the casing;        a second member for driving the spindle in translation along its axis relative to the casing, the second member for driving in translation being screwed onto a threaded portion of the spindle so that the spindle advances or returns along the axis as a function of the relative rate of rotation of the drive members.        
A tool of that type is known, for example, from U.S. Pat. No. 5,351,797. The drive mechanism of such a tool is referred to as “having positive feed” in English or “à avance mécanique” in French.
A single motor thus ensures, via the drive mechanism, that the spindle is driven in rotation about its axis and, at the same time, that it is advanced or returned by translation along its axis.
Since the translation and rotation drives of the spindle are connected mechanically, the advance of the spindle per revolution is constant. Thus, the variations in the speed of the motor have no effect on the rate of advance per revolution. The thickness of the swarf formed therefore remains constant and promotes the surface quality and the precision of the bores drilled by such a tool.
Although such a tool is found to be generally satisfactory, its use for drilling deep bores or materials renowned for being difficult, such as composite materials, may prove to be tricky.
For example, when a deep bore is drilled, the swarf accumulates in the flutes of the drill bit carried by the spindle until the motor of the tool is caused to stall. It is then necessary to proceed in several stages or to start fresh drilling cycles for the same bore to be drilled.
The drilling quality may also be degraded if the drill bit is repositioned incorrectly relative to the bore which has been started. The pressure of the swarf inside the bore also causes the deterioration of the surface thereof.
In order to solve that problem, French Patent No. 2,873,315 has proposed a tool provided with means for displacing the second member for driving in translation along the axis of the spindle between an advanced position and a returned position. Thus, in the course of drilling, the spindle is removed completely from the bore under the action of the displacement means in order to evacuate the swarf and to ensure that the bore is cleared. Although such a withdrawal of the spindle can be effected far more rapidly than by the conventional return movement of the spindle, it increases substantially the time necessary to drill a bore.
In addition, the performance of the clearing operation requires a control system in order to observe a predetermined clearing frequency or to meet a specific clearing requirement. Such a control system is relatively complex.